A variety of substances acting as greenhouse gases are well known, and according to the Kyoto protocol adopted on 11 Dec. 1997 in Kyoto, Japan, commitment was made to reduce carbon dioxide, methane, nitrous oxide, perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6).
A process for treating greenhouse gases including carbon dioxide may be largely divided into capturing and fixation. Fixation may include chemical fixation, electrochemical fixation, and biological fixation, etc.
The biological fixation used in the present invention may directly fix carbon dioxide emission sources using suitable aquatic plant such as microalgae, microphytes, seaweeds, see grasses, and the like, resulting in a simple process. Also, the biological fixation may have a variety of industrial applications, for example, the resulting biomasses may be converted to feedstocks or specific chemical substances may be extracted from microorganisms. Currently, leading developed countries are studying practical applications of carbon dioxide fixation by biological methods, however it is very difficult to establish the design and operating conditions of reactors for fixation of carbon dioxide emitted from industry.
Conventionally, technologies for treating carbon dioxide using aquatic plant and/or algae merely comprise supplying, to aquatic plant and/or algae such as microalgae, microphytes, seaweeds, see grasses, and the like, carbon dioxide emitted from power generators, iron works, factories, and the like. It is theoretically possible to remove carbon dioxide using aquatic plant and/or algae, but practically problematic by the following reasons:
Firstly, as a waste gas mixture includes greenhouse gases, and also substances of strong toxicity such as sulfur, carbon oxide, and the like, all aquatic plant and/or algae do not live and grow on greenhouse gases. This is because toxic substances in the waste gas mixture, such as sulfur, carbon oxide, and the like, kill aquatic plant and/or algae. To remove greenhouse gases in the waste gas mixture, aquatic plant and/or algae strong against toxic substances in the waste gas mixture should be used. However, a theory that aquatic plant and/or algae feed on greenhouse gases is currently discovered, but any information about aquatic plant and/or algae strong against toxic substances is not well known. In these circumstances, removal of greenhouse gases using aquatic plant and/or algae is an undeveloped field.
Secondly, deposits of dead microphytes or microalgae may be particularly stuck on a culture container during mass cultivation of aquatic plant and/or algae, so that the quality of cultivated aquatic plant and/or algae may be deteriorated. Also, a transparent culture container may become opaque, which will interrupt the light, and consequently, photosynthesis of aquatic plant and/or algae may be hindered. To solve this problem, a great amount of additional costs may be required.
Thirdly, a variety of bacteria live in a culture container or a pond as well as aquatic plant and/or algae. However, there is no solution to prevent propagation of bacteria, resulting in reduced harvesting efficiency and deteriorated quality of aquatic plant and/or algae.
Fourthly, since it needs to harvest aquatic plant and/or algae having adopted carbon dioxide for removal of carbon dioxide, it requires to harvest aquatic plant and/or algae in a culture container. However, because microphytes and microalgae among aquatic plant and/or algae have a very small size between about 5 and about 40 μm, it is not easy to filter and harvest only microphytes and microalgae in a culture container. Moreover, it is not preferred to keep cultivating aquatic plant and/or algae in a culture container. This is because carbon dioxide may be emitted in a culture solution again when aquatic plant and/or algae having adopted carbon dioxide deteriorate or die, thereby failing to remove carbon dioxide.
Fifthly, a culture solution has a high production cost, and thus, it takes a great amount of costs to cultivate aquatic plant and/or algae.
Recently, with a rapid increase in oil price, interests in new renewable energy increases. In particular, bioenergy, one of new renewable energy, becomes the center of attention as alternative energy of fossil fuels causing global warming. Among bioenergy technologies, bioenergy using aquatic plant and/or algae absorbs greenhouse gases and converts them into energy, and thus, it is considered as effective in reducing carbon dioxide (CO2) and securing energy.
Many energy dependent countries are in urgent need of development of alternative energy. In these circumstances, importance of bioenergy is becoming higher.